Method for the production of superabsorbers

Abstract

A process for producing superabsorbents, comprising polymerization, drying, crushing, pneumatic conveying, comminuting and classifying, wherein the gas temperature at the end of the pneumatic conveying is from 50 to 95° C.

The invention claimed is: 
     
       1. A process for producing superabsorbents by polymerizing a monomer solution or suspension comprising
 a) an ethylenically unsaturated monomer which bears an acid group and is at least partly neutralized, 
 b) at least one crosslinker, 
 c) at least one initiator, 
 d) optionally an ethylenically unsaturated monomer copolymerizable with the monomer mentioned under a), and 
 e) optionally one or more water-soluble polymer, comprising 
 i) polymerizing the monomer solution or suspension to provide a polymer gel, 
 ii) optionally comminuting the polymer gel obtained in step i), 
 iii) drying the polymer gel obtained in step i) or step ii) in an air circulation belt drier with multiple heating zones and at least one cooling zone to provide a dry polymer gel, 
 iv) crushing the dried polymer gel obtained in step iii) to provide polymer particles, 
 v) optionally coarsely comminuting the polymer particles obtained in step iv), 
 vi) pneumatically conveying the polymer particles obtained in step iv) or step v), 
 vii) optionally removing incompletely dried polymer particles from the polymer particles obtained in step vi), and further processing the remaining dried polymer particles in step viii), step ix), or step x), 
 viii) optionally classifying the polymer particles obtained in step vi) or step vii), and sending a coarse fraction to step ix) or step x), 
 ix) optionally intermediately storing the polymer particles obtained in step vi), step vii), or step viii), 
 x) comminuting the polymer particles obtained in step vi), step vii), step viii), or step ix), 
 xi) optionally pneumatically conveying the polymer particles obtained in step x), 
 xii) classifying the polymer particles obtained in step x) or step xi), and 
 xiii) optionally surface postcrosslinking the classified polymer particles obtained in step viii) and/or step xii), 
 wherein a gas temperature at the end of the pneumatic conveying in step vi) is from 50 to 95° C., and 
 a cooling output of the at least one cooling zone in step iii) is used to adjust the gas temperature at the end of the pneumatic conveying in step vi). 
 
     
     
       2. The process according to  claim 1 , wherein the polymer particles obtained in step vi) or the polymer particles remaining in step vii) after the removal of the incompletely dried polymer particles are classified in step viii), and the coarse fraction is sent to step ix) or step x). 
     
     
       3. The process according to  claim 1 , wherein the gas temperature at the end of the pneumatic conveying in step vi) is from 59 to 80° C. 
     
     
       4. The process according to  claim 1 , wherein a dwell time of the polymer particles between the end of the drying in step iii) and the end of the pneumatic conveying in step vi) is less than 30 minutes. 
     
     
       5. The process according to  claim 1 , wherein a moisture content of the polymer particles in step x) is from 1% to 10% by weight. 
     
     
       6. The process according to  claim 1 , wherein the dried polymer gel is crushed in step iv) by means of a spiked roller or a cross-blade comminutor. 
     
     
       7. The process according to  claim 1 , wherein the polymer particles are comminuted in step x) by means of at least one multistage roll mill. 
     
     
       8. The process according to  claim 1 , wherein the polymer particles are classified in step viii) and/or step xii) by means of at least one tumbler sieving machine. 
     
     
       9. The process according to  claim 1 , wherein the polymer particles obtained in step iv) are coarsely comminuted in step v) by means of at least one roll crusher. 
     
     
       10. The process according to  claim 1 , wherein the polymer particles obtained in step x) are conveyed pneumatically in step xi). 
     
     
       11. The process according to  claim 1 , wherein incompletely dried polymer particles are separated in step vii) from the polymer particles obtained in step vi). 
     
     
       12. The process according to  claim 1 , wherein the polymer particles obtained in step vii) are classified in step viii). 
     
     
       13. The process according to  claim 1 , wherein the polymer particles obtained in step viii) and/or step xii) are surface postcrosslinked in step xiii). 
     
     
       14. The process according to  claim 1 , wherein the polymer particles have a centrifuge retention capacity of at least 15 g/g. 
     
     
       15. The process according to  claim 1 , wherein the gas temperature at the end of the pneumonic conveying step (vi) is from 53 to 90° C. 
     
     
       16. The process according to  claim 1 , wherein the gas temperature at the end of the pneumonic conveying step (vi) is from 56 to 85° C.